The systemic and spinal administration of morphine and other opioid agents produces a well-defined analgesia in a variety of mammals, including humans. To date, studies of opioid analgesia have been performed solely in experimental models using mammalian species. Given the mandate to decrease potential pain and distress in experimental animals used for biomedical research, the development of an adjunct or alternative model for pain research using non-mammalian vertebrates is desirable. The proposed studies employs a novel behavioral model for the assessment of opioid analgesia using the common grass frog, Rana pipiens. The amphibian central nervous system is the prototype of the vertebrate nervous system and the frog isolated spinal cord preparation in vitro is used extensively in neurophysiological and pharmacological investigations. The results of the proposed studies will provide much needed comparative data on nociceptive processing, the organization of endogenous opioid systems, and the phenomena of opioid tolerance and dependence in a non-mammalian vertebrate species. The proposed method, called the acetic acid test (AAT), allows for the measurement of threshold in unanesthetized frogs by applying dilute concentrations of acetic acid onto the hindlimb. The nociceptive threshold is estimated by the concentration of acid that produces a wiping response within 5 sec of application. Importantly, this model uses a noxious chemical stimulus which activates C-fiber polymodal nociceptors in the frog skin; a specific nociceptive pathway not examined in commonly-used in pain models. The overall goal of the laboratory is to gain a basic knowledge of opioid analgesia, opioid tolerance and dependence, and the distribution of opioid receptors in the amphibian CNS. The specific aims are to: 1) characterize the relative potency of systemically administered opioid agonists in producing antinociception 2) characterize the relative potency of intraspinally administered opioid agonists in producing antinociception 3) determine the spinal opioid receptors mediating the antinociceptive effects of spinally administered opioids using highly-selective opioid agonists and antagonists, 4), characterize the opioid binding sites in the amphibian CNS using highly-selective opioid radioligands and receptor autoradiography techniques, and 5) investigate the development of opioid tolerance and the expression of opioid dependence in chronically-treated animals. The expected results will help to establish the first comparable non-mammalian model for assessment of analgesic efficacy and for pain research. Finally, the proposed research may lead to a greater understanding of opioid analgesia and the mechanisms of opioid tolerance and dependence.